1. Field of the Invention
The present invention is directed to a high intensity discharge (HID) lamp driving circuit, and in particular, to a power factor improving power converter (PFIPC) for use with the HID lamp driving circuit.
2. Discussion of Background and Other Information
Electronic lighting control devices for use with HID lamps are known in the art. Examples of such devices include, for example, U.S. Pat. No. 5,942,859 (corresponding to Japanese Laid-Open Patent Application Nos. HEI 10-294188 and HEI 10-294191); U.S. Pat. No. 5,962,981 (corresponding to Japanese Laid-Open Patent Application Nos. HEI 10-294186 and HEI 10-294189); and U.S. Pat. No. 5,932,976. Such prior art lighting control devices require many large size components, which makes it difficult to minimize manufacturing costs and minimize the size of the device.
Another problem associated with such prior art lighting control devices relate to an in-rush current that results when a capacitor associated with a boost converter is initially connected to an electrical power source. Specifically, when a discharged capacitor is initially connected to a power source, the capacitor behaves like a short-circuit. Thus, a large flow of current (e.g., the in-rush current) occurs, which can damage electrical components. In order to reduce the occurrence of this in-rush current, such prior art devices typically employ an in-rush current protection device (such as, for example, a soft start switch) that limits, for a predetermined period of time, the amount of current that is passed when the electrical power is initially connected to the lighting control device. Unfortunately, the inclusion of the in-rush current protection device further increases the complexity of the circuit design, and further increases the cost of manufacture and size of the lighting control device.
A still further problem associated with the prior art light control devices pertain to a switching loss associated with the employed switching elements. Switching elements that are turned ON/OFF at a high frequency rate lose a significant amount of power for switching, in addition to a conduction loss. The switching loss can be calculated by multiplying the applied current and applied voltage in a transient state (e.g., when the switching element is turned ON/OFF). The switching loss becomes larger as the switching frequency becomes higher. Prior art devices employ a plurality of switching elements that are turned ON/OFF at a high frequency (e.g., several 10's kHz) rate in the lighting state. Because each switching element exhibits a switching loss, the total amount of the switching loss associated with the prior art devices are equal to the sum of the switching loss of each switching element, which becomes fairly large. Due to the large power loss and resultant low circuit efficiency, the prior art devices are unable to operate with a high efficiency, which is necessary to achieve a compact lighting device.
It is noted that in the prior art lighting control devices, such as, for example, U.S. Pat. No. 6,426,597, switching elements associated with a polarity reversing circuit are turned ON and OFF at a high frequency rate in both the non-lighting (e.g., ignition) state and the lighting state. As noted above, switching losses are related to the frequency rate at which the switching elements are operated.
This large switching loss results in a further disadvantage of the prior art lighting control devices. Namely, the large switching loss associated with the prior art switching elements is converted into heat. The temperature of each component of the lighting control device must be maintained below some predetermined critical temperature level. However, when the switching loss is large, the power loss is large. The lost power (which is discharged as heat), increases (raises) the temperature of the components in the lighting control device. While a heat sink may be installed to dissipate the heat, the inclusion of heat sinks further raises manufacturing costs and increases the physical size of the device.